1. Field of the Invention (Technical Field)
The present invention relates to photovoltaic solar cells for the generation of electrical power directly from light, whether natural sunlight or artificial light, and more particularly, to solar cells, and methods for making solar cells, comprising contacts which are recessed into the front and/or back surfaces of the cell.
2. Background Art
Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
In a typical “buried-contact” silicon solar cell, the current collection grid is recessed in grooves on the front surface. By minimizing the surface area occupied by the grid contacts (i.e. grid obscuration), there is more area available for current collection. However, even though the surface contact area is less, series resistance losses do not increase because the contact area increases with respect to the depth of the contact and there is a larger cross-sectional area for the conductor. Other advantages of buried-contact cells include a heavy diffusion that is only in the buried-contact groove (reduces contact resistance and losses due to recombination of electrons and holes at the contact) and the contact metallization is selectively deposited only in the grooves. Buried-contact cells, and methods for fabricating such cells, are described in, for example, U.S. Pat. Nos. 4,726,850 and 4,748,130. High-efficiency large-area buried-contact cells have been demonstrated on both single-crystal and multicrystalline silicon substrates.
A representative process sequence for fabricating a buried-contact cell is as follows:                1. Alkaline etch        2. Light phosphorus diffusion (60 to 100 Ω/sq)        3. HF etch        4. Deposit silicon nitride on front or both surfaces        5. Laser scribe and etch grooves in front surface        6. Heavy phosphorus diffusion in grooves (<20 Ω/sq)        7. Deposit aluminum on rear surface        8. Alloy aluminum through rear dielectric layers        9. HF etch        10. Deposit thin layer of Ni in grooves by electroless plating        11. Sinter Ni layer        12. Deposit Cu in grooves by electroless plating        
As shown in FIG. 1A, a buried contact solar cell made of a silicon substrate 10 according to the prior art method comprises light phosphorus diffusion 12 over the illuminated surface, dielectric layer 18 deposited or thermally grown over the front surface, and grooves 20 subsequently applied. After fabrication of grooves 20, as shown in FIG. 1B heavy phosphorus diffusion 30 is applied, such as by means of a gaseous diffusion preferably using phosphorus oxychloride (POCl3), phosphine (PH3), phosphorus tribromide (PBr3) or another gaseous phosphorus precursor, and an aluminum layer is applied and alloyed in a subsequent step to form an aluminum-alloyed junction 50 on rear surface of the cell. Thereafter, the heavily diffused grooves 20 are filled with metal, such as by electroless deposit of Ni thin layer 42, followed by sintering of the Ni layer and subsequent electroless deposit of Cu layer 40. The final structure, as shown in FIG. 1C, results in grooves with heavily doped (e.g. with heavy phosphorus diffusion) inner surfaces 30 to lower contact resistance and contact recombination, and metal grids or contacts 40, 42. Alternatively, a silver (Ag) metal paste may be applied to the heavily doped grooves, which is subsequently fired, as disclosed in U.S. Pat. No. 4,748,130.
Prior art buried-contact cells have a number of advantages, including a light phosphorus diffusion over the illuminated surface for high collection efficiency, a heavy phosphorus diffusion inside the grooves for low contact resistance and low contact recombination, and self alignment of the heavy phosphorus diffusion and electroless metallization to the grooves. There are some simple variations on the prior art methods, such as using a dicing saw or diamond saw rather than laser scriber to cut the grooves (although laser patterning provides finer line geometries). The primary disadvantage of the prior art process sequence is the relative complexity, time, and expense of the process. It would be advantageous to eliminate some process steps, for example the second diffusion of gaseous phosphorus into the grooves, to realize the same or an improved device structure. Further, eliminating electroless plating would be advantageous, since electroless plating involves use of hazardous chemicals that require stringent controls over waste-water treatment.
The use of self-doping metal contacts on the surface of solar cells has been described by Meier et al., U.S. Pat. No. 6,180,869. See also Daniel L. Meier, et al., “Self-doping contacts to silicon using silver coated with a dopant source,” 28th IEEE Photovoltaic Specialists Conference, pg. 69 (2000). In order to make a doped silicon layer and simultaneously provide a contact, the Ag:dopant paste can be placed directly on the silicon surface or fired through a silicon nitride layer, although in the latter case the paste must contain components which dissolve the nitride (see M. Hilali, et al., “Optimization of self-doping Ag paste firing to achieve high fill factors on screen-printed silicon solar cells with a 100 ohm/sq. emitter,” 29th IEEE Photovoltaic Specialists Conf., New Orleans, La., May 2002). However, the self-doping metal contact methods have only been applied to surfaces, and suffer from high shadowing losses due to spreading out of the self-doping paste on the surface of the cell.